1. Field of the Invention
The present invention relates to a non-aqueous electrolyte secondary battery.
2. Description of the Related Art
Non-aqueous electrolyte secondary batteries which charge and discharge by transfer of lithium ions from the negative electrode to the positive electrode and vice versa are studied and developed enthusiastically. Such a non-aqueous electrolyte secondary battery is expected as large power sources for electric cars and hybrid cars using a combination of an engine and electric motor from the viewpoint of environmental problems. Also, these non-aqueous electrolyte secondary batteries have attracted considerable attention as power sources for devices other than automobiles.
For these non-aqueous secondary batteries used as large power sources, such characteristics that large current can be accumulated momentarily are regarded as more important than the characteristics required for power sources used for small mobile telephones and note-type personal computers.
A lithium ion secondary battery is known as one of these non-aqueous electrolyte secondary batteries. Almost all lithium ion batteries which are commercially available at present use graphite-type materials as the negative electrode material. These graphite-type materials enable high-capacity and long life batteries to be attained. However, when large current is momentarily input to or output from these secondary batteries, metal lithium is easily precipitated on the surface of graphite, giving rise to safety problems.
In order to solve the above problem, a non-aqueous electrolyte secondary battery using a lithium-titanium composite oxide having a higher Li absorption potential than graphite as the negative electrode active material has been put into practical use. The lithium-titanium composite oxide is known to have longer life than graphite-type materials because it is decreased in volume change along with charge-discharge reactions. Among these materials, a lithium-titanium oxide having a spinel structure is very promising.
On the other hand, M. Koltypin et al, “On the Stability Of LiFePO4 Olivine Cathodes under Various Conditions (Electrolyte Solutions, Temperature)” Electrochemical and Solid-state Letters, 10 (2) A40-A44 (2007) discloses a non-aqueous electrolyte secondary battery improved in high-temperature characteristics, rapid charging characteristics and output characteristics by using a lithium-titanium composite oxide as the negative electrode material and a lithium-cobalt oxide (LiCoO2) as the positive active material. The lithium-cobalt oxide is a layered compound and has stable cycle characteristics. However, because this positive electrode active material contains cobalt which is a rare metal, it is an obstacle to economical production. Therefore, an attention is focused on a positive electrode active material excluding cobalt, for example, lithium-iron phosphate (LiFePO4).
A non-aqueous electrolyte secondary battery using the positive electrode active material excluding cobalt and a lithium-titanium composite oxide as the negative electrode active material has high harmony with the environmental and is also superior in large-current characteristics. Therefore, an attention is focused on this non-aqueous electrolyte secondary battery as a large power source including power sources for electric cars and hybrid cars.
However, much residual water exists in the lithium-titanium composite oxide. For this reason, when, particularly, lithium-iron phosphate is used as a positive electrode active material, this residual water dissolves the iron compound of the lithium-iron phosphate, resulting in remarkably deteriorated cycle characteristics.